Abstract

We evaluate strategies by which a VOIP provider might select PSTN gateways through which to terminate calls. Different gateways may have different blocking probabilities and prices for terminating VoIP calls to the PSTN . A customer placing a call to a VoIP provider is impatient and may hang-up if the delay in setting up the call is large. Possible strategies for the VOIP provider are to route each call to the gateway that charges the smallest price, or to simultaneously route each call to a set of gateways, a strategy called `forking’. Forking creates a race between gateways (only one of which ultimately connects the call). It reduces the average call setup delay, but it increases the average price of terminating a call and the load on the system. We investigate some problems of choosing an optimal set of gateways to which to fork a call request. We discuss the effects of forking on the overall call blocking probability, and the incentives for gateways and VoIP providers to deploy it. By analysing a multi-player game we see that is can be advantageous for all concerned if gateways introduce a small signalling charge. Along the way we discuss the following problem of “optimal re-dialing”. Suppose you are trying to minimize the expected time until you get through to a switchboard. If your first attempt is answered by a busy signal, should you redial immediately, or might it be better to wait a short time between successive attempts?